The present invention relates to a device for protecting a line of a network of electricity supply lines, and to a protection process implemented by means of such a device.
In such a network, the occurrence of a fault could cause disturbances which might affect the operation of the network and the quality of the supply provided to subscribers.
For example, an improperly eliminated fault on a very high voltage network, that is to say a network carrying a voltage of 150 kilovolts, 225 kilovolts or 400 kilovolts could result in a considerable part of the geographical zone covered by the network being deprived of supply. Likewise, faults on a high voltage network, namely a network carrying voltages of the order of 42 kilovolts, 63 kilovolts or 90 kilovolts give rise to voltage dips or short, or possibly long cuts in the supply provided to subscribers.
With the aim of limiting the effects of a fault, the networks are generally fitted with protective relays each arranged at one end of a line of the network. These relays are equipped with means for detecting faults and are each linked to a circuit breaker adapted for disconnecting the line from the network of lines as soon as a fault occurs on the line.
Such protective relays must exhibit extreme reliability, that is to say an absence of operating fault and an absence of inadvertent operation.
Indeed, a malfunction of a protective relay is constraining, for the network, if it leads to late and non-selective elimination of faults, that is to say if it causes the opening of a circuit breaker which is not necessary for the elimination of a fault.
Indeed, high voltage networks and very high voltage networks are generally intended to carry high energy levels, the load currents possibly reaching several thousand amperes.
After a relay opens, there is an instantaneous diversion of load onto the portions of the network remaining in service.
It is therefore necessary to trigger only the protective elements which are strictly necessary for the elimination of a fault, so as not to be confronted with power levels which are insupportable on other parts of the network, and consequently with cascade triggerings.
The selectivity of elimination of faults is a major constraint for protective relays, in so far as it considerably increases the complexity of the relays.
To achieve the objectives mentioned above, various types of protective devices are nowadays fitted to electricity distribution networks.
One of these types of protective device, which are universally used by the operators of distribution networks, uses the technique of so-called xe2x80x9cdistance protectionxe2x80x9d.
According to this technique, each protective relay is adapted for zonewise monitoring of the network, several relays possibly being tripped by one and the same fault.
More particularly, according to this technique, each line is fitted with a pair of relays each arranged at one end of the line, so that each relay is adapted for monitoring a first zone of the network extending over a portion of the line and corresponding to a zone of certain detection of a fault on this zone and at least one second zone of the network juxtaposed with the first zone and overlapping a portion of an adjacent supply line, the second zone encompassing a portion of the line in which the other protective relay is situated and corresponding to a zone of uncertain detection of a fault on this line.
Thus, for example, the first zone covers 80% of the length of the line, and the second zone covers a portion of the network extending between 80% and 120% of the length of the line, a third zone possibly being provided onwards of a portion of the network extending from 120% of the length of the line.
In this type of protective device, a fault detected in the first zone causes instantaneous opening of the circuit breaker associated with the corresponding relay, whilst a fault detected within the other zones gives rise to a delay in the opening of the circuit breaker of this relay.
This is because the locating of the site of the fault is carried out with a measurement error which may be as much as 20%.
Thus, when a relay detects a fault in the first zone, it locates the fault in a certain manner within the supply line corresponding thereto and causes instantaneous opening of the circuit breaker.
On the other hand, a fault detected in the second zone may either be situated on the supply line to which the relay belongs, or on the adjacent supply line.
Consequently, in this case, the relay performs a time-out so as to be able to determine whether the protective relay fitted to the adjacent supply line has detected this fault in the first zone and has thereupon caused the opening of the circuit breaker with which it is fitted.
On the other hand, if the fault, detected in the second zone, is situated on the same supply line, the other relay, that is to say the relay arranged at the opposite end of the line, has necessarily detected the presence of this fault in the first zone and has caused the opening of the circuit breaker with which it is fitted.
In this case, it is however necessary to cause the two circuit breakers to open so as to isolate the incriminated portion of the network and thus avoid retention of the supply, which might create risks for the environment and for personnel (fires, electrocutions, etc).
Certain types of distance protection devices are equipped with linking means ensuring transmission of information between the relays of each pair so as to cause the opening of the circuit breaker of each relay in response to a detection, by this relay, of a fault in the second zone and to a detection, by the other relay, of a fault in the first zone.
This technique, although effective for isolating a portion of defective network, nevertheless requires that cabling and means for sending and receiving specific data be provided on the supply network, thereby considerably increasing the cost of such a protective device.
The purpose of the invention is to alleviate these drawbacks.
Its subject is therefore a device for protecting a line of a network of electricity supply lines, comprising at least two protective relays each arranged at one end of the line and each furnished with means for detecting faults and a circuit breaker associated with each relay and adapted for disconnecting the line from the network of lines in the presence of a fault detected on this line, each protective relay being adapted so as to monitor a first zone of the network extending over a portion of the line and corresponding to a zone of certain detection of a fault on the line and at least one second zone of the network, juxtaposed with the first zone and overlapping a portion of an adjacent supply line, the second zone encompassing a portion of the line in which another protective relay is situated and corresponding to a zone of uncertain detection of a fault on the said line, characterized in that at least one of the relays comprises means for detecting the opening of the circuit breaker associated with the other relay in response to a detection, by the latter, of a fault in the first zone so as to cause the opening of the circuit breaker associated with the said at least one relay, and in that the said means for detecting the opening of the said circuit breaker comprise means for calculating a characteristic representative of the variation of a measurement value of the impedance of the line and means for comparing the said characteristic with at least one threshold value for triggering the opening of the circuit breaker.
The protective device according to the invention can furthermore comprise one or more of the following characteristics, taken in isolation or according to all the technically possible combinations:
the calculating means consist of means for calculating the following ratio R1:       R    1    =            "LeftBracketingBar"                        Z          Bclos                -                  Z          Bopen                    "RightBracketingBar"              "LeftBracketingBar"              Z        Bclos            "RightBracketingBar"      
xe2x80x83in which:
ZBclos represents the value of the impedance measured by one of the relays, for a defective phase of the line, when the circuit breaker associated with the other relay is closed, and
ZBopen represents the value of the impedance measured by one of the relays, for a defective phase of the line, when the circuit breaker associated with the other relay is open;
the calculating means consist of means for calculating the following ratio R2:       R    2    =                    "LeftBracketingBar"                  Z          Bclos                "RightBracketingBar"            -              "LeftBracketingBar"                  Z          Bopen                "RightBracketingBar"                    "LeftBracketingBar"              Z        Bclos            "RightBracketingBar"      
xe2x80x83in which:
|ZBclos| represents the absolute value of the value of the impedance measured by one of the relays, for a defective phase of the line, when the circuit breaker associated with the other relay is closed; and
|ZBopen| represents the absolute value of the impedance measured by one of the relays, for a defective phase of the line, when the circuit breaker associated with the other relay is open;
the calculating means consist of means for calculating the impedance of the line.
The subject of the invention is also a process for protecting a line of a network of electricity supply lines, the line being equipped, at each end, with a protective relay comprising means for detecting faults and with a circuit breaker adapted so as to disconnect the line from the network of lines in the presence of a fault detected on the latter, the process comprising the steps consisting in, for one of the relays at least:
monitoring a first zone of the network extending over a portion of the line corresponding to a zone of certain detection of a fault on the said line and at least one second zone of the network, juxtaposed with the first zone and overlapping a portion of an adjacent line, in such a way as to detect a fault in the said zones, the second zone encompassing a portion of the said line in which the other relay is situated and corresponding to a zone of uncertain detection of a fault on the line; and
opening the circuit breaker of the relay should a fault be detected in the first zone,
characterized in that in the course of the step of monitoring the first and second zones of the network, the value of a characteristic representative of the variation of a measurement value of the impedance is calculated, the calculated value is compared with at least one threshold value for detecting the opening of the circuit breaker of the other relay, and the opening of the circuit breaker of the relay is triggered should the threshold value or values be exceeded.
According to a particular mode of realization of this process, with the said characteristic consisting of the impedance measured after the occurrence of the fault, the threshold values constitute, in a complex representation, a zone of detection of the opening of the circuit breaker of the said other relay, which zone is delimited, for a positive transit of power, by straight lines respectively corresponding to minimum and maximum values of the real and imaginary parts of the impedance measured in the absence of a fault, and to maximum and minimum arguments of the impedance measured when the distant circuit breaker is open, and by a straight line corresponding to a variation in the impedance measured as a function of the resistance of the fault.
According to another mode of realization of this process, with the said characteristic consisting of the measured impedance, the threshold values constitute, in a complex representation, a zone of detection of the opening of the circuit breaker of the said other relay, which zone is delimited, for a negative transit of power, by straight lines corresponding to minimum values of the real and imaginary parts of the impedance of the line measured after the occurrence of the fault and to minimum and maximum arguments of the variation in the measured impedance when the said distant circuit breaker is open, and by a straight line computed on the basis of the value of the impedance in the presence of a fault of zero resistance at the same spot and of the variation in the impedance measured as a function of the resistance of a fault of non-zero resistance.